What does Genesis 18:27 mean?

In the prior verses, Abraham brazenly questioned if God's judgment on Sodom would be fair, since it might mean harming righteous people along with wicked people. In this verse, Abraham seems to have a moment of clarity, and checks himself. Abraham acknowledges to the Lord that he is being very bold even in speaking to Him. After all, Abraham is a mere man. He calls himself "dust and ashes." He recognizes, apparently, that he has no right to demand anything from God.

This is a key part of our perspective on God and His actions. God most certainly does what is good, fair, and just—and for that reason, human beings who are limited have no right to assume He is being unfair or unjust simply because we don't like His decisions. God's conversation with Abraham, regarding the people of Sodom, is meant to clarify this very idea. Abraham worries that God might do something unfair. God's response proves that His planned action against Sodom and Gomorrah is perfectly, absolutely justified.

Abraham's moment of humility will not prevent him, however, from continuing to push the Lord for assurances that He will spare the city of Sodom for the sake of any righteous people who live there. This almost certainly is Abraham's attempt to keep his nephew Lot, who lives in Sodom, from being destroyed.